1. Field of the Invention
This invention relates in general to wellhead assemblies, and in particular to a production tree with multiple safety barriers against excessive pressure.
2. Description of the Prior Art
One type of wellhead assembly particularly in a subsea well includes a wellhead housing located at the upper end of conductor pipe. Casing hangers for supporting the casing land in the wellhead housing. After the well has been drilled to total depth, a Christmas tree is lowered onto and connected to the wellhead housing.
A tubing hanger lands in the tree in one type, called a horizontal tree. The tubing hanger is secured to a string of tubing that extends into the well for producing well fluids. The tubing hanger has an axial passage for production fluids, and a lateral passage extending from it that registers with a lateral passage in the tree.
During installation of the tubing and tubing hanger and other operations, it may be necessary to circulate well fluid between the interior of the tubing and the tubing annulus. The horizontal tree has a first tubing annulus passage that extends from the tubing annulus to a port on the exterior of the tree. A second tubing annulus passage extends from the exterior port back into the bore of the tree above the tubing hanger seals. A flow line is connected to the external port for delivery of fluids to and from the tubing annulus. Both the first and second tubing annulus passages in the tree have hydraulically controlled valves for opening and closing the tubing annulus passage. A internal tree cap is typically installed in the bore of the tree above the tubing hanger.
The upper end of the second tubing annulus passage may join the bore of the tree between the tubing hanger and the internal tree cap, or it may lead into the tree bore above the internal tree cap. The junction of the second tubing annulus passage with the tree bore allows communication with a riser during installation and workover. Normally, the riser connects to an exterior profile on the tree. After removal of the internal tree cap, an inner riser, such as a drill string or tubing, will be run through the outer riser and stabbed into the tubing hanger to communicate with the interior of the string of production tubing. The tubing annulus passage communicates with the annular space in the tree bore surrounding the inner riser. A choke and kill line alongside the outer riser normally provides a flow path from the surface platform to the annular space in the tree bore.
To meet safety requirements, two safety barriers are required for each passage in a wellhead assembly that may be under pressure. For the tubing hanger production passage, a removable plug is installed in the axial passage of the tubing hanger above the lateral passage to provide one safety barrier. In the prior art, typically the internal tree cap provided the second safety barrier. While workable, an internal tree cap requires a large seal that is fairly expensive. Also, if the second tubing annulus passage leads into the bore above the internal tree cap, there would be only one safety barrier in the tubing annulus above the lateral production port. If gas is being injected into the external port of the tubing annulus for gas lift purposes, a good practice would require an additional safety barrier.
U.K. patent application GB 2346630 discloses two removable plugs in the tubing hanger above the lateral passage. The upper plug could comprise a second safety barrier, eliminating the need for an internal tree cap that seals. The patent application discloses a test port that leads from the space between the plugs for monitoring leakage past the lower plug.
The wellhead assembly of this invention has a production tree or wellhead with a bore into which a tubing hanger lands and is sealed by a tubing hanger seal. A first portion of a tubing annulus passage extends upward through the tree to an exterior port. The port is adapted to be connected to a source of gas for injection into the tubing annulus. A second portion of the tubing annulus passage extends upward from the exterior port into the bore of the tree. A valve is located in the first portion of the tubing annulus passage. Two closure members are located in the second passage, providing two safety barriers for the exterior port if used to inject gas.
At least one of the closure members is a valve, preferably the lower one. The upper closure member is a removable plug in one embodiment. The plug extends into the portion of the second tubing annulus passage where it enters the bore. The plug may be accessible by an ROV through a corrosion cap that lands on the production tree.
Alternately, the upper closure member may be a shuttle type valve that is located in an axial portion of the second tubing annulus passage within the sidewall of the tree mandrel. In one embodiment, this valve is a check valve that is biased upward to a closed position. In the closed position, a stem of the valve protrudes above a rim of the production tree. When a riser connector lands on and connects to the tree, the riser connector will move the stem downward, opening the second tubing annulus passage into the bore of the tree.
The internal tree cap may be eliminated as a second safety barrier for the production passage in the tubing hanger. The second safety barrier could be provided by a removable plug located in the axial passage of the tubing hanger. Venting is provided by a vent passage that leads to an exterior of the tubing hanger. A mating passage extends through the tree and mates with the vent passage of the tubing hanger.
If desired, a secondary locking mechanism may be mounted above the tubing hanger to prevent an upward movement of the tubing hanger in the event the tubing hanger primary lockdown fails. The secondary lockdown would not require any seals for sealing to the bore of the tree. The secondary lockdown may also be utilized to connect with a small diameter wireline riser. The wireline riser engages a neck on the secondary lockdown and has a stinger that extends into sealing engagement with the axial passage in the tubing hanger.